docs: def has_equivalent(self, equiv):
          """
          Check to see if this unit object as an equivalent unit in *equiv*.
  
          Example
          -------
          >>> from unyt import km
          >>> km.has_equivalent('spectral')
          True
          >>> km.has_equivalent('mass_energy')
          False
          """
          try:
              this_equiv = equivalence_registry[equiv]()
          except KeyError:
              raise KeyError('No such equivalence "%s".' % equiv)
          old_dims = self.dimensions
          return old_dims in this_equiv._dims